A known fuel cell, as presented in FIG. 1, can include an anode side 100, a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs), oxygen is fed to the cathode side 102 and reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through electrolyte material 104 to the anode side 100 where it reacts with the used fuel to produce water and carbon dioxide (CO2). Between the anode 100 and the cathode 102 is an external electric circuit 111 including a load 110 for the fuel cell.
In FIG. 2 is presented a SOFC device as an example of a high temperature fuel cell device. SOFC devices can utilize for example natural gas, bio gas, methanol or other compounds containing hydrocarbon mixtures as fuel. The SOFC device system in FIG. 2 can include multiple fuel cells in one or more stack formations 103 (SOFC stack(s)).
A larger SOFC device system can include many fuel cells in several stacks 103. Each fuel cell includes an anode 100 and cathode 102 structures as presented in FIG. 1. Part of the used fuel may be recirculated in a feedback arrangement 109. The SOFC device in FIG. 2 also includes a fuel heat exchanger 105 and a reformer 107. Heat exchangers are used for controlling thermal conditions in the fuel cell process and there can be more than one heat exchanger in different locations of a SOFC device. The extra thermal energy in circulating gas is recovered in one or more heat exchangers 105 to be utilized in a SOFC device or externally. Reformer 107 is a device that converts fuel, such as natural gas, to a composition suitable for fuel cells, such as a composition containing all or at least some of the following: hydrogen, methane, carbon dioxide, carbon monoxide, inert gases and water. In each SOFC device, a reformer is optional.
By using measurement means 115 (such as fuel flow meter, current meter and temperature meter), measurements for the operation of the SOFC device are carried out. Only part of the gas used at the anodes 100 is recirculated in the feedback arrangement 109 and the other part of the gas is exhausted 114 from the anodes 100.
Fuel cells are electrochemical devices for converting chemical energy of reactants directly to electricity and heat. Fuel cell systems have the potential to significantly exceed the electrical and CHP (Combined production of Heat and Power) efficiency of traditional energy production technologies of comparable size. Fuel cell systems are widely appreciated as a desirable future energy production technology.
In order to maximize the performance and lifetime of fuel cell systems, accurate control of the fuel cell operating conditions is desired. Fuel cells produce DC current, whereas in higher power systems, AC output can be desired and thus a power conversion from DC to AC is involved. To allow for practical interfacing and current collection from the fuel cells and subsequent power conversion, the fuel cells are manufactured as stacks containing several individual cells connected in series.
In fuel cell systems including several stacks, the electrical interconnection topology of the stacks can be a design parameter. Series connection of several stacks provides for lower cabling and power conversion losses as well as lower cost for components. Electrical isolation limitations as well as the desired operating voltage level of the fuel cell load can, however, limit the feasible amount of stacks to be serially connected. Hence, if higher power levels are desired (e.g., more than what can be achieved with a single string of serial connected stacks) some sort of parallel connection of stacks or groups of stacks can be involved.
When electrical sources such as fuel cells are connected in parallel, uneven load sharing may occur if there are deviations in the electrical characteristics of the individual sources. With fuel cells, this can be an issue since uneven load sharing may reduce the efficiency (due to reduced fuel utilization) and/or significantly deteriorate those fuel cells operating above the average current. Due to inherent variances in series resistance between stacks as well as variations due to age, temperature etc, uneven load sharing to some extent can be expected if stacks are connected directly in parallel. Electrical parallel connection of stacks can be an issue in high temperature fuel cell systems due to intrinsic negative temperature coefficient of their internal resistance. This characteristic can give rise to positive feedback behaviour in the load sharing balance between parallel connected stacks; i.e., a stack with higher current heats up, which tends to increase the current further due to decreased internal resistance. To avoid the current sharing issues, separate converters for each stack or series of stacks have been used, bringing a considerable higher cost to the system.
Fuel cells can have current-voltage characteristics which are far more flexible than that of a battery. Exemplary shape and full operating range 128 of the fuel cell (and for comparison an exemplary shape and full operating range 130 of a battery) are presented in FIG. 3. As seen from FIG. 3, the voltage level at the nominal operation point 124 of the fuel cell can be significantly lower than the maximum voltage of the fuel cell achieved in no- or low-load conditions. Reference number 130 represents the nominal operation window of the battery and reference number 126 represents the nominal operation point of the battery, whereas number 128 depicts the operating window for a fuel cell. As a consequence, power electronics, or more generally loads, interfacing with the fuel cells can deal with a comparatively wide operating voltage window. Fuel cell degradation occurring over the lifetime of the system further reduces voltages at full load involving an even wider operating window. For power electronics, a large voltage window can involve several compromises in terms of component selection and filter dimensioning, both negatively affecting price and efficiency. For example, if the nominal operating window for a fuel cell is about 0.6-0.8V/cell and the open circuit voltage is about 1.1 V/cell, power electronic components can be dimensioned according to the highest voltage although operated most of the time at a lower voltage.
Large fuel cell systems can incorporate a 3-phase inverter for feeding power to a grid or 3-phase load. For an inverter, an optimum input voltage (e.g., DC-link voltage) can be a minimum voltage at which undistorted output can be generated. The theoretical minimum voltage can be the main voltage multiplied by sqrt(2) (e.g., 566V for a 400 VAC grid connection). On top of this voltage, a reasonable voltage margin is desired to compensate for voltage drops in filters, switches and grid voltage variations. An exemplary DC-link setpoint for a 400 VAC inverter can be 625V. Operation above this voltage yields higher switching and filtering losses and higher electromagnetic emissions, whereas operation below this voltage may result in a distorted output. The maximum allowable voltage is determined by the voltage rating of inverter components, for example, 800V for a 400 VAC inverter whereby the maximum voltage, applying an exemplary safety margin of, for example, 20% is 720V.
Feeding power from a fuel cell to an inverter can be carried out by direct connection to the inverter (or other load) or by feeding the fuel cell power through a DC/DC converter stage. If a DC/DC converter is used then the voltage window matching can be done on the DC/DC side and inverter voltage kept optimum at most or all times. Separate DC/DC converters can be used for different groups of fuel cell stacks, whereby each DC/DC converter can control the current of the corresponding stack group to mitigate uneven current sharing issues common for parallel connection of stacks. Introducing the separate DC/DC converters can lead to conversion losses and additional economical cost associated with separate converters.
If fuel cells are connected directly to the load (e.g., inverter) then their voltage (e.g., number of cells), can be chosen such that the output voltage is sufficient even with a minimum output voltage from the fuel cells (e.g., maximum load). Assuming a minimum cell voltage of 0.65, 961 cells are used to produce an inverter voltage of 625V. At no-load conditions with a cell voltage of about 1.1V, the output voltage is 1058V. This involves the use of at least 1200V rated components in the inverter, for example, even higher voltage rating rather than 800V components. Furthermore, the inverter operates at optimum voltage only at end of life conditions. These compromises can add costs and decrease efficiency of the inverter. A lack of active means exists to control the current sharing between stacks connected in parallel. Particularly high temperature fuel cells are susceptible to uneven current sharing in such cases when their series resistance has a negative temperature coefficient causing differences in current sharing. The stack or group of stacks providing a higher load may be overstressed and suffer accelerated non-reversible degradation.